1. Field of the Invention
The present invention relates generally to RF impedance measurement and in particular to RF impedance measurements using two point voltage sampling without a phase detector. Some embodiments also relate to adjusting an impedance matching network after the measurement.
2. Description of Related Art
Mobile handsets such as cellular phones are being manufactured using higher levels of integration and use in broader frequency band coverage. As a result, the performance limits of embedded antenna technology are being stretched. Variations in load impedance at the antenna due to environmental changes such as the position at which the phone is held, the frequency band being used and other contributors create a mismatch or increased voltage standing wave ratio (VSWR) at the antenna port. In addition, the body effects of a head or hand near the antenna contribute to capacitive loading which also results in an impedance mismatch. This can lead to a shift in antenna center frequency and an increased VSWR mismatch. In addition to reception problems, any mismatch will further result in a reduction in power radiated from the antenna.
FIG. 1 depicts a prior art RF system which includes an adaptive impedance matching network. The system includes an RF power amplifier PA 20 having an output coupled to the input of a duplexer 22. The duplexer directs the RF signal from amplifier 20 to the system antenna 24 by way of the adaptive impedance matching network 26. Duplexer 22 further channels RF signals received on the antenna to a system receiver (not depicted). The adaptive impedance matching network includes a tunable impedance matching network 30 which matches the impedance of the antenna to some target value that matches that of the duplexer. Typically, an antenna impedance has a real component Rant ranging from 30 to 100Ω and a reactive component j Xant of 0 to +100 jΩ. The matching network converts the antenna impedance to some target impedance such as an impedance matching that of the coupler 28.
The antenna impedance Zant can change, as previously noted, due to a change in the physical environment surrounding the antenna. The impedance at the input of the tunable matching network 30 is monitored by periodically measuring the amplitude of the RF voltage at the input and output of the directional coupled 28 using respective peak detectors 32A and 32B. The phase relationship between the two detected voltages is measured using a phase detector 34. The peak voltage measurements and the phase measurement are then provided to a processing device 36 such as a digital signal processor to compute the impedance. In the event the measured impedance differs from the target impedance due to a change, by way of example, in the antenna characteristics, the processor adjusts the tunable matching network 30 as needed to return to the target impedance.
The above-described approach requires an impedance sensing section which is separate from the impedance matching section. In addition, a phase detector is used. A phase detector having good accuracy and low current is difficult to achieve over the 690 Mhz to 2690 Mhz range of interest in many cell phone applications. As will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings, an RF impedance improved detection scheme is disclosed which does not rely upon a phase detector and which does not require a sensing element separate from the matching network.